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Abstract

Selective Laser Melting (SLM), a powder based Additive Manufacturing (AM) process, has gained considerable attention in the aerospace, biomedical and automotive industries due to its many potential benefits, such as, capability of fabricating complex three-dimensional components, shortened design to product time, reduction in process steps, component mass reduction and material flexibility. This process uses metallic powder and is capable of fabricating complex structures with excellent microstructure which make SLM not only an improvement over other manufacturing processes but also innovative material processing technology. Inconel 625, a nickel-based super alloy is widely popular in aerospace, chemical and nuclear industries. This alloy is characterized by having high tensile, creep and rupture strength and is widely used because of its excellent fatigue and good oxidation resistance properties. However, excessive tool wear and low material removal rate make it difficult to manufacture by conventional machining methods at room temperature. Selective laser melting, therefore, becomes a good solution for complex Inconel 625 parts. The formation of constituent phases of this alloy is a function of process parameters such as local temperature, hold time at temperature, local cooling rate and local compositions in the melt-pool. The effect of each process parameter on the resulting microstructure and mechanical properties must be understood in order to properly control the machines and predict the properties of the parts being fabricated. Therefore, the aim of the research work is to investigate the effect of key process variables of SLM systems (the EOS M270 Powder-bed system at The University of Louisville) on the melting response and solidification microstructure of Nickel based super-alloy Inconel 625. The effect of processing parameters on Inconel 625 was investigated on single track deposits and bulk deposits. Multiple combinations of laser power and scan speed were used to fabricate the deposits by selective laser melting (SLM). Surface morphology and dimensions of the single track deposits were characterized using optical and SEM microscopy. To evaluate the geometrical feature of the melt pool, the cross-section of the single track deposits was studied. The result was then utilized to develop a process parameter map which is insightful to identify the optimum parameters that produce high-density parts. Beside laser power and scan speed, scan pattern plays an important role in controlling microstructural features. Therefore, a careful study of scan pattern is important to understand microstructural evolution during SLM. In this study, two types of scanning pattern (Alternating and Rotating) were used to build samples of Inconel 625. Microstructure differences due to different scan patterns in as-built Inconel 625 samples were then studied in detail. In both cases, the grains were observed to grow preferentially in the build direction, but there were also clear effects of grain orientation differences due to scan direction effects. The tensile properties were compared with respect to laser scanning pattern, build orientation and post-processing heat treatment. Results show that although different scanning pattern produces distinctively different microstructures, its effect on the tensile property was not significant. However, tensile property anisotropy was observed with respect to the build orientation. The horizontally built samples showed relatively higher tensile strength as compared to the vertically built samples. Although the tensile strength decreased after heat-treatment, it was still comparable to the standard wrought processed ones. Fractography on the tensile tested samples showed ductile fracture characteristics. Investigation on fatigue behavior of Inconel 625 by SLM process was performed with respect to build orientation and post-manufacturing heat treatment. The study revealed the anisotropic behavior of Inconel 625 where horizontally built samples showed superior fatigue property than vertically built samples. The lower fatigue lives of vertical samples are primarily because of the presence of voids and un-melted particles located near the surface of the samples. In general, fatigue life of Inconel 625 by SLM improved after heat treatment. The improvement is attributed to the formation of coarser grains after heat treatment. The study provides a comprehensive understanding of the microstructure and mechanical properties of Inconel 625 manufactured by SLM process which fills an immediate need of the metal AM community.